It is known that by applying beta-rays the basis weight or grammage of a moving paper web can be measured. Furthermore, it is known that by simultaneously applying beta-radiation and a characteristic x-radiation to the moving paper webs the total ash and filler content can be measured, albeit on the condition that either there is only one single filler in the paper web, namely kaolin (clay), talcum or a filler component containing pure CaCO.sub.3. In the case of filler component mixtures containing kaolin, talcum, CaCO.sub.3 and/or TiO.sub.2, the total ash content measurements require the filler component recipe, i.e. the relation between the individual filler components.
In the above conventional measurement technique a natural radioactive Fe-55 radiation source is used as source for the characteristic x-radiation. The Fe-55 radiation source emits a quantum radiation with a mono-chromatic energy of 5.9 keV. The known filler-measurement technique makes use of the fact that, by using the mono-energetic measurement ray from the Fe-55 radiation source, a strictly exponential absorption law with clearly-defined absorption coefficients applies. This means that all the radiation extinction contributions of the various components of the paper web tested add up mathematically to a resulting extinction signal.
Furthermore, a measurement method using fluorescence x-ray analysis to measure the filler content in paper webs is known. With this method, a filler component mix containing Mg, Ca, Al and Ti can be measured. Use is made in this regard of the fact that each of the elements Mg, Ca, Al and Ti has a specific fluorescence wavelength. The content of each of these elements in the paper web tested is determined by measuring the specific fluorescence of each element per unit of time. This analytical method is the standard method applied nowadays in instruments for the laboratory analysis of samples but, to date, it has not caught on for on-line application because of serious apparatus-related problems. The problems are basically: the temperature sensitivity of the scintillation meter for the detection of the fluorescence radiation; measurement of radiation in scatter mode with its attendant problems of errors caused by changes in the position of the web in the measuring gap.
On the other hand, it is known from the German Pat. No. 2,910,673 that the absolute content or the basis weight of a substance in a moving web can be measured with a high degree of accuracy by applying infra-red rays and beta-rays, as long as both the substance to be measured and a further substance which forms a major component of the web each have a distinct absorption band in the infra-red radiation range and the substances in the material of the web are mixed, i.e. form a stock mix. This well-known measuring method is used especially to determine the absolute water content in paper webs which have cellulose as their main substance. The measurement in the infra-red radiation range is done with the help of a measurement wavelength which is absorbed by the water, a measurement wavelength absorbed by the cellulose and a reference wavelength which is not appreciably absorbed by either the water or the cellulose. By comparing the radiation intensity measured with the measurement wavelength for the water with the radiation intensity of the reference wavelength, an extinction signal is obtained which represents a measure of the absolute water content in the paper web. This measurement signal for the absolute water content is, however, distorted by a lengthening of the infra-red measuring ray path caused by the opacity (internal scatter) of the paper. Therefore, to determine the true absolute content of the water in the paper web, the measurement result obtained is multiplied by a correction value which takes account of the structural characteristics of the mixture of water and cellulose in the web. This correction value is determined by comparing the measurement result for the entire basis weight of the paper web obtained using beta-rays with the measurement signal for the basis weight of cellulose obtained using infra-red rays. The determination of this correction factor is based on the assumption that the distortion of the measurement signal for the basis weight of the water, which is obtained from infra-red measurement, is proportional to that of the measurement signal for the cellulose obtained using infra-red measurement and that the measurement result for the total basis weight of the web, obtained using beta-rays, corresponds to the true basis weight of the cellulose because of the low percentage share of water in the total basis weight. Also of importance in this connection is the fact that the measurement by means of beta-rays is not, of course, influenced by the structural characteristics of the web.
In the known state of the art according to German Pat. No. 2,910,673, two reference wavelengths are generally used in the infra-red radiation measurement technique to eliminate wavelength-dependent losses of infra-red rays. With the help of both the reference wavelengths, it is possible to determine a corrected reference value for the measurement signal obtained with each measurement wavelength.
The four-wavelengths measurement method according to German Pat. No. 2,910,673 is not only intended for use in measuring the water content in paper but in general for measurements on paper webs in which the content of a substance in the material of the web is to be measured, provided that the substance to be measured is mixed with the other substances in the web and that, like the substance to be measured, a further substance which can give rise to a false or distorted measured value has also got a distinct absorption band for infra-red radiation. Thus the known measurement method could also be used to measure the content of a filler component in a web, as long as the filler component has a distinct absorption band in the infra-red radiation range. However, the problem arises that in the case of filler components present in the paper web, the approximation that the total basis weight of the web measured with beta-rays corresponds to the true basis weight of the cellulose no longer applies. Furthermore, when, in addition to a first filler component, a second filler component is selected to be present in the material mix, one of the two filler components does not generally have a distinct absorption band in the infra-red radiation range. The measurement method according to German Pat. No. 2,910,673 is thus not suitable for the selective filler measurement of moving webs containing two filler components.
In the paper industry there is a general pressing need for measuring equipment which can determine the distribution of two filler components, e.g. kaolin (clay) and calcium carbonate, in moving webs of filled or coated paper. This need exists especially when waste paper is added in an unregulated way to the paper pulp used to produce the paper web, or when broke of pigment-coated paper is recycled into the raw paper pulp and an exact knowledge of the filler composition gets lost in the process.
On the other hand, the fillers used for the manufacture of paper generally contain compounds of metals such as aluminum in kaolin and satin-white, calcium in calcium-carbonate and satin-white, titanium in titanium dioxide and barium in barium sulphate. Such metals have, of course, characteristic absorption characteristics in a radiation energy range of several keV which are conducive to the highly selective measurement of the relevant individual fillers, essentially independent of the filler composition--that is in the presence of the base material cellulose and also moisture contained in the web.
These characteristics of the fillers form the basis for the measurement method using the x-ray fluorescence analysis which has already been mentioned above.
In principle, the problem of selective filler measurement could also be solved for example by taking two separate measurements for two different fillers present in the stock-mix and then by solving a system consisting of two linear equations with two unknown quantities which represent the content of each of the two fillers respectively. The ability to adjust the energy of an x-ray which emits a continuous radiation appears to offer a solution to the problem in question. Two sets of filler-measuring equipment with suitably adjusted energy spectra could be used. The one set of measuring equipment could be adjusted to the energy range at the resonant point of the one filler component and the other to the energy range at the resonant point of the other filler component. In the resonance range, namely, the relative absorption coefficients differ most from each other, so that, as far as selectivity is concerned, one would obtain optimal conditions.
The emission spectrum of an x-ray source emitting continuous radiation is, however, extremely broad. The half-band-width of the spectrum amounts to about one third of the maximum energy. Due to the broad spectrum of the continuous radiation the absorption coefficient of the substance to be measured changes with increasing weight as a higher proportion of the low energy radiation is absorbed. This means that the absorption law as a function of a higher order would have to be empirically determined. The absorption law of filler measuring equipment which uses continuous radiation would, therefore, be dependent on the system. The advantage of the adaptability of the radiation energy in the case of filler measuring equipment which uses continuous radiation would, therefore, be completely cancelled out if the equation system contained non-linear terms. Furthermore, there is the problem of instability of the measurement as a result of drift in the high-voltage supply of the x-ray tubes.